1. Field of the Invention
The present invention relates to paper coating slips containing, as a binder, a copolymer which is obtainable by free radical polymerization of ethylenically unsaturated compounds, wherein at least one of the ethylenically unsaturated compounds is a polymer having at least one copolymerizable ethylenically unsaturated group, a number average molecular weight of from 500 to 50000 g/mol and at least one carboxyl group (referred to below as ethylenically unsaturated polymer for short).
Paper coating slips essentially comprise pigment and binder. The binder is intended to fix the pigments to the paper and to ensure cohesion in the coating obtained.
In the printing process, for example in an offset printing press, strong tensile forces act on the coated paper (paper coat) owing to the high viscosity of the printing ink. The resistance which the paper coat offers to these forces is referred to as pick resistance. A distinction is made between dry pick resistance and wet pick resistance. The wet pick resistance is important particularly in aqueous offset printing since, in the second printing unit, the printing ink comes into contact with a water-moist paper, and the paper coat must have sufficient binding power under these conditions.
2. Description of the Background
In order to increase the pick resistance, the polymers generally contain acid groups. Such polymers are described, for example, in WO 97/00776.
Emulsion polymers which have ethylenically unsaturated polymers having a plurality of acid groups as components form the subject of WO 95/04767.
In the case of paper coating slips known to date, the binding power of the binder and hence the pick resistance are still insufficient.